# Full text of "Text Book Of Mechanical Engineering"

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```Dimensions and W. G.                       -277

that described at page 58. The spaces are first cut on .tire angle
with a spur-wheel cutter, and the finish given with-the hob by
placing both wheel and worm in position, as at Fig. .72, and
rotating the latter on a milling spindle. (See App. //.,/.* 823.)

Dimensions.—In most workshops the inch is divided into
vulgar fractions in the common way. But in dealing with work
of great accuracy, or where small differences are to be repre-
sented, the above divisions have to be carried beyond sixteenths,
and then become cumbersome. To avoid this difficulty, the
decimal system of division has been used for a considerable
period in a few shops, and has proved a great boon, being easily
learnt by any workman, and its advantages greatly valued. We
have spoken of high and low gauges for interchangeable work.
Where these are used, the drawings are supplied with what are
known as 'plus and minus' dimensions. Thus all shafts, pins,
&c., are figured "002" larger than the size required, an inch pin
being 1*002"; and holes are marked "005" larger than their
pins, an inch pin requiring a hole 1*007". There,is an under-
stood plus and minus allowance of "002" on both these dimen-
sions, so that if a large pin and small hole come together, there
will be a minimum clearance of *ooi", while a small pin in a
large hole will have a maximum clearance of -009". For driving
fit, the hole and shaft are figured the same, and the kind of fit
noted. (See Appendix //.,/. 825.)

It was long ago found advisable to fix the thickness of thin
plates and the diameter of small wires by reference to a table of
numbers, which received the name of the Birmingham Wire
Gauge or B. W. G., and where each number had a corresponding